The invention relates generally to providing desired optical properties at a surface of a light transmissive member, and relates more particularly to providing an antireflection coating onto the surface of a substrate, such as a display screen.
Coatings are applied to light transmissive members in order to impart desired optical properties to the members. For example, one or more coatings may be applied to a screen of a computer monitor in order to provide polarization or to reduce reflection. One method for reducing light reflection from a substrate, such as a cathode ray tube (CRT), is to coat the surface with an antireflection layer having a thickness of approximately one-quarter wavelength of the light spectrum of interest. The antireflection layer may be an organic material, such as a polymer, or an inorganic material, such as a metal fluoride, where the deposited layer has a refractive index that is less than that of the CRT. A high reduction in reflection is achieved when the refractive index of the deposited antireflection layer generally equals the square root of the refractive index of the CRT material. This approach has limitations. Single layer, low refractive index antireflection coatings often exhibit distinct colorations. In addition, easily sputtered materials with suitably low indices are not readily available.
Antireflection coatings having wider reflection suppression bandwidths, and consequently less coloration in reflection, may be obtained by using multiple deposited layers. Three common antireflection coating designs are the quarter/quarter (Q/Q), the quarter/half/quarter (Q/H/Q), and the quarter/quarter/quarter (Q/Q/Q) stacks. That is, the antireflection coating is formed of a number of layers having selected refractive indices, with the layers being approximately equal in optical thickness to one-quarter (Q) or one-half (H) of a wavelength of interest. Typically, the wavelength is in the visible spectrum of light (e.g., 550 nm).
In order to provide an efficient antireflection coating, the refractive indices of the particular layers must be considered. One well known approach is to alternate between a layer having a high refractive index and a layer having a low refractive index. An alternative approach is to provide a three-layer stack in which the layer closest to the substrate has a refractive index that is below the refractive index of the center layer, but greater than the refractive index of the outermost layer (i.e., Q/Q/Q: medium/high/low). One concern with either of these approaches is that the antireflection performance (i.e., reflection amplitude and bandwidth) is dependent upon achieving the desired refractive indices for the individual layers. For example, in the three-layer stack, the medium refractive index of the first layer may be difficult to properly tailor when using conventional materials. A second concern is that when readily sputtered materials such as indium oxide, zinc oxide or tin oxide are incorporated into the antireflection coatings, the coatings are not clear in transmission throughout the visible light spectrum.
In the three-layer stack, the selection of high index materials that meet the design goals of the center layer are somewhat limited. Titanium oxide (TiO2) is the most common high index material. This material has a refractive index in the range of 2.3 to 2.6. However, from a practical stand-point, as a result of the higher attainable deposition rate, niobium oxide (Nb2O5) is often preferred, despite its refractive index being in the range of only 2.2 to 2.4. Generally, the low index material of the outermost layer is a sputtered silicon oxide (SiO2). This is primarily due to the ease with which silicon oxide is deposited using AC powered cathodes. The most common and easily sputtered medium index materials are zinc oxide (ZnO), indium oxide (InO) and tin oxide (SnO). When sputter deposited, these oxides tend to have a slightly higher refractive index than desired. Moreover, the resulting optical stack tends to be yellow in transmission.
What is needed is a method of fabricating a relatively inexpensive antireflection coating that has a wide reflection suppression bandwidth and is clear in transmission. What is further needed is such an antireflection coating that is relatively easy to manufacture.
A multi-layer antireflection coating is formed to include a sequence of a medium refractive index layer, a high refractive index layer, and a low refractive index layer, with the medium refractive index layer being hydrogenated in order to reduce its refractive index to below 1.99. Preferably, the hydrogen concentration (atomic concentration) within the medium refractive index layer is at least 7 percent. More preferably, the hydrogen concentration exceeds 9 percent, and most preferably 18 percent. Of the three layers, the medium refractive index layer is closest to the substrate for which antireflection is achieved. Preferably, this layer is formed of an indium, tin or zinc oxide or an oxide of an alloy in which indium, tin or zinc is a major constituent (i.e., greater than 50 percent by weight). The hydrogen is introduced to the material to achieve two advantages. Firstly, the hydrogen reduces the refractive index of the layer, thereby improving the reflection suppression bandwidth of the antireflection coating. Secondly, introducing the hydrogen into the medium index layer causes this layer to be clearer in transmission. That is, a yellow-looking coating is less likely to result.
The invention is primarily focused on an antireflection coating that consists of only three layers, although hardcoat layers, adhesion layers and lubricating layers can be added to the antireflection coating. The preferred low index layer is silicon oxide, which has a refractive index of 1.44 to 1.51. The preferred materials for the high index layer are titanium oxide or niobium oxide. Most preferably, the material is niobium oxide (Nb2O5), since it has a higher deposition rate. The index of Nb2O5 is in the range of 2.2 to 2.4. Regarding the medium index layer, it has been discovered that by adding the hydrogen when sputtering one of the preferred materials, the oxides become very clear in transmission and the refractive index is reduced to a more desirable range. Specifically, depending upon how much hydrogen is added, the refractive indices of the hydrogenated preferred materials are reduced from the range of 2.05 to 2.1 to the range of 1.85 to 1.95. The preferred index for the medium index layer is 1.85 to 1.98. More preferably, the index of the medium index layer is in the range of 1.90 to 1.95.
Although the hydrogen-modified oxide coatings are preferably used to provide advantages in the three-layer antireflection stack, the invention may be used in other antireflection coatings. That is, the hydrogenation of one or more layers to tailor the refractive index and/or to achieve desired clarity may be utilized in other approaches to fabricating an antireflection coating.
An advantage of the invention is that the antireflection coating is readily sputtered. AC or DC sputtering may be utilized. Moreover, the sputtering may be carried out using fully oxidized reactive sputtering or using transition mode techniques. The transition mode provides rate enhancement by utilizing computer-controlled sputtering in which the reactive gas availability relative to the availability of the sputtered metal atom is carefully controlled. The monitoring of the availabilities may be achieved by means of monitoring the plasma emission, the plasma impedance, or the oxygen partial pressure. The control of the relative availabilities may be achieved by controlling the reactive gas flow of the system or by controlling the power to the target. The target may be metal or ceramic.